An electrochromic (“electrochromic” is hereinafter sometimes abbreviated as “EC”) material is widely known as a substance which undergoes changes in optical absorption properties (absorption wavelength and absorbance) through an electrochemical redox reaction. An electrochromic device (EC device), which utilizes the EC material, is applied to a display apparatus, a variable reflectance mirror, a variable transmission window, and the like.
As the EC material, there are widely known both an inorganic EC material and an organic EC material. Of those, an organic EC material, in particular, a low-molecular-weight organic EC material has the following feature: transparency in its uncolored state and a high absorbance in its colored state can both be achieved and appropriate molecular design can be performed through estimation of an absorption wavelength of the colored state. Meanwhile, in an electrochemical reaction, such high energy as to cause decomposition of a molecule itself of an organic compound is applied to the organic EC material in some cases. Accordingly, deterioration of the organic compound itself caused by an electrochemical redox reaction of the compound is often perceived as a problem in putting an EC device using the organic EC material into practical use. In view of this, many attempts have been made to improve durability of the material itself by solving the problem.
In PTL 1, there is disclosed an organic EC device having its durability improved by using an organic low-molecular-weight compound as an EC molecule and adjusting a concentration balance of the compound.
Hitherto, there has also been known an organic EC device containing, in addition to the EC material included in the organic EC device, a substance which undergoes a redox reaction under a certain potential condition.
In PTL 2, there is disclosed an organic EC device containing: an anodic electroactive material; a cathodic electroactive material; an additive which is more easily reduced than the cathodic electroactive material; and an additive which is more easily oxidized than the anodic electroactive material.
In addition, in PTL 3, there is disclosed an organic EC device containing three or more kinds of electroactive materials. Therein, at least two kinds of materials out of the electroactive materials of the organic EC device of PTL 3 are EC materials, and by virtue of these electroactive materials, a pre-selected perceived color can be maintained throughout a normal range of voltages of the organic EC device. In addition, in PTL 3, the perceived color utilizes a staging phenomenon in which the most easily oxidized electroactive material dominates a tint of the perceived color, and when oxidation (reduction) potentials of the electroactive materials are uniformized, their redox reactions are allowed to progress in the same potential region. In addition, the organic EC device of PTL 3 can produce the pre-selected perceived color through the uniformization of the oxidation (reduction) potentials of the electroactive materials.
However, the organic EC device of PTL 1, though exhibiting an improving effect on durability when subjected to a low-speed potential scan operation, has low durability when a large current is flowed transiently in order to obtain a high response speed.
The additives contained in the organic EC device of PTL 2 are added in order to prevent a residual color of the cathodic electroactive material or the anodic electroactive material, but when the additives are EC materials, reactions of the additives cause different coloration. In addition, the additives to be introduced into the organic EC device of PTL 2 are compounds which are more easily reduced or oxidized in terms of potential than the anodic electroactive material or the cathodic electroactive material. Accordingly, when the additives are added in more than necessary amounts, a charge amount required for initiation of coloring/bleaching of the organic EC device is consequently increased, which may cause an increase in power consumption in driving the device or a decrease in response speed.
Further, in the organic EC device of PTL 3, the materials to be used have almost the same redox potential, and hence during oxidation or reduction of a predetermined material, any other material is also oxidized or reduced competitively, resulting in a problem in that power to be consumed in the oxidation or reduction reaction is increased.